1. Field of the Invention
The present invention relates generally to cement grouts and methods for use of the same. More particularly, the present invention relates to a pumpable cement grout for grouting of annular cavities.
2. Background Art
Much of the infrastructure of modern society is in need of repair or upgrading, or, at the least is in need of means for reducing the frequency of such required repairs or upgrading. For example, it is well known that a great many sewer systems and bridges throughout the United States and elsewhere are greatly in need of upgrading or frequent repair.
One relatively new and effective technique that has been used for upgrading sewer systems involves relining existing concrete sewer lines with plastic liner pipe, such as high density polyethylene (HDPE) pipe. The plastic liner is inserted into the sewer line, and is pulled or jacked longitudinally through the sewer line into place. In order to fit within the concrete sewer line, however, the plastic liner pipe must necessarily have an outside diameter which is smaller than the inside diameter of the concrete sewer pipe, so as to provide clearance between the two. Accordingly, an annular cavity is formed between the concrete sewer pipe and the plastic liner pipe. Concrete sewer pipes may typically range in size from an inside diameter of 15 inches or less, to an inside diameter on the order of 60 inches. Accordingly, it will be understood that it is typically at least very difficult, and usually virtually impossible, for personnel or construction equipment to enter a sewer pipe to install or backfill a plastic liner.
While, as noted, the plastic liner pipe necessarily has an outside diameter which is somewhat less than the inside diameter of the concrete sewer pipe, so as to permit movement of the liner pipe within the sewer pipe, it is not desirable that the liner pipe be very much smaller in diameter than the sewer pipe, lest the capacity of the upgraded sewer system be severely reduced. Accordingly, plastic liner pipes are typically installed so that there is adequate, but minimal clearance between the outside of the liner pipe and the interior surface of the sewer pipe; in a typical installation, the radial clearance (i.e. the radial extent of the annular gap) may range from on the order of 3 inches to less than 1 inch.
Once the plastic liner pipe has been pulled or jacked into place, and the annular space between the old pipe and the new liner has been formed, it is typically desirable or necessary to fill the annular space with grout to protect the liner from future damage. The potential for such damage may exist, for example, when the old concrete pipe is badly deteriorated and there is a danger that portions of the wall of the pipe may fail. A material which has been found effective in providing such protective grouting is concrete grout which, in conjunction with the inexpensive plastic liner pipe, provides the potential for old sewers to be rehabilitated with minimal excavation and cost. As noted above, however, direct access to the interior of the sewer line by personnel and construction equipment is typically difficult or impossible; this precludes the possibility of grouting of the pipe liner by means of the conventional cement grouting techniques which are used for the backfilling of large tunnels and similar structures, which typically involve boring holes through the wall of the tunnel at a multiplicity of points along the length of the tunnel, and then injecting cement grout through the holes and into each local area of the cavity about the tunnel.
Because of such inability to apply proven conventional techniques, grouting of the plastic pipe liners has been attempted by injecting a cement grout into the annular cavity at a first point in the pipe so that the grout flows longitudinally through the pipe towards a second point in the pipe. A very serious problem has, however, been encountered when attempting to grout the plastic liners by flowing the cement grout longitudinally through the sewer pipe, because the plastic liner pipe itself is typically unable to resist the significant external pressures which are exerted by the injected concrete grout. As noted above, the volume of space between the plastic liner pipe and the existing sewer pipe is typically small; hence, it is typically very difficult to maintain a low grout pressure when injecting the grout longitudinally through the annular cavity. The plastic liner pipe can easily collapse under such injection pressure, some pipes being unable to withstand external pressures as low as 3 pounds per square inch (psi). For example, a plastic liner pipe which is commonly used for lining sewer pipe is high density polyethylene (HDPE) pipe having a wall thickness of SDR 32.5 (where SDR is the ratio of outside diameter-to-wall thickness). HDPE pipe having a wall thickness of SDR 32.5, while able to contain significant internal pressures, has a tendency to collapse within one day if subjected to an external pressure in excess of 4 psi. Another, less commonly used size of HDPE pipe has a wall thickness of SDR 26, and tends to collapse when subjected to an external pressure in excess of 8 psi. Such external pressure maximums can easily be exceeded when using the typical longitudinal injection techniques described above to inject conventional cement grouts. The conventional cement grouts generally exhibit fairly high viscosities, and relatively high injection pressures are thus required to force the grouts along the annular cavity. Furthermore, as the grout moves through the annular cavity, the grout tends to hydrate (i.e. bind or set up due to the chemical reaction between the portland cement and water in the grout), particularly if the concrete sewer pipe and/or plastic liner are dry, thus compounding the difficulty of maintaining a low grout injection pressure. These problems become critical when long distance are encountered between sequential injection points, in other words, when the individual runs to be grouted between access points are fairly long. This is often the case in conventional city sewers, where the distance between access points provided by manholes may often be on the order of 300 to 500 feet. In some such cases, the contractors performing such grout work, if using conventional cement grouting materials as described above, have resorted to drilling additional access holes vertically through the pavement and soil overlying the sewer pipe so as to provide additional grout injection points, which is obviously a time-consuming and costly makeshift approach.
In the event that the plastic liner collapses during the grout injection, however, the consequences may be catastrophic for the job. Not only is the flow of sewer water through the sewer line blocked by such a collapse, which may result in an overflow, but it is also frequently necessary to then excavate and replace the entire section of sewer line in which the liner is collapsed, at great expense. Accordingly, there exists a need for a grout material, and method for use thereof, which is both effective and inexpensive, yet which reduces the possibility of collapse of the plastic liner pipe during grouting of the liner in a sewer line.
Another significant group of structures within the national infrastructure which are often in need of repair or upgrading are our bridges. In particular, a number of such bridges use wire or cable stays to support the bridge structure. Such stays typically run from a bottom anchor to a tower, from which the bridge span may be supported. Most typically, each individual wire stay comprises a wire or strand bundle having a multiplicity of individual wires therein. Such wires are very frequently manufactured of a high tensile strength steel, which will quickly corrode if exposed to weather. Accordingly, if such wires remain exposed to the elements, burdensome maintenance expenses may be incurred. A number of attempts have been made to enclose such wire bundles in protective sheathes. As with the sewer liner pipes described above, such protective sheathes may be polyethylene pipe; when such plastic pipe, such as high density pipe is installed about the wire bundle, an annular cavity is formed between the external pipe and the internal wire bundle.
A number of attempts have been made to grout or otherwise fill the annular space between the polyethylene pipe and the wire bundle, in large part to prevent any water which may penetrate the pipe from coming into contact with the wire bundle. Such attempts have encountered a number of significant difficulties. For example, it will be understood that such wire stays typically tend in a somewhat vertical direction; accordingly, one end is typically at a much higher elevation than the other end. When grouting of the wire stays is performed, the grout is injected through the pipe at a relatively low point along the stay, and rises upwardly through the pipe along the stay, so as to achieve the desired uniformity and freedom from air gaps. As the top of the column of grout in the pipe proceeds to greater heights, greater and greater static head pressures will be generated. This, in combination with the friction generated by the flow of the relatively viscous conventional grout through the pipe and around the wire bundle and the hydration of the grout, as described above, will necessitate greater and greater injection pressures to continue the injection of the grout. In extreme cases, the injection pressure may reach a point where the polyethylene pipe sheath may rupture. Alternatively, the sheathed wire stay may be grouted in relatively short stages so as to avoid development of excessive injection pressures; performing the grouting in such relatively short stages, however, is quite inefficient, and compounds the time and cost required to perform the job. Furthermore, conventional non-foamed grout mixes impose undesirably large weight loads on the cable stays, and, when cured, exhibit only minimal shock absorption capabilities for such applications.
Accordingly, there exists a need for a lightweight grout suitable for use in filling an annular cavity intermediate a wire bundle of a wire bridge stay and a sheath about the bridge stay so as to provide effective protection for the wire bundle.